The invention disclosed in this application relates to recording using an array of recording devices called "array elements". The most common recording array element in use today is the laser diode. For the purposes of this application, the terms "array element" and "diode" are used interchangeably; however, use of the word "diode" should not limit the invention as it is intended to apply to all arrays of recording devices regardless of the nature of the array element itself.
Laser diodes have been used in many prior art recording techniques as have monolithic laser diode arrays. Monolithic laser diode arrays used in recording typically contain 10-100 diodes and the recording is done with either photonic exposure or thermal exposure. Photonic systems react to the total exposure to photon energy, such that each photon striking the recording surface helps to expose it. Conversely thermal systems respond to peak temperatures and must reach a certain threshold for exposure to occur. Thermal systems usually operate in the IR, while photonic systems usually operate in the visible or UV range, but either system can operate in any range of the spectrum. Each diode may be a single mode source or a short multi mode stripe and is said to record a particular "track" or "raster line" on the recording surface. Note that throughout this application the terms "track" and "raster line" are used interchangeably. Diode arrays can contain anywhere from 10 to 1000 diodes. In typical printing applications, the tracks on the recording surface are spaced between 10 and 20 microns apart, but for data storage applications, the tracks can be as close together as 0.5 microns in order to permit high density recording.
A current problem associated with the use of diode arrays is the diode spacing within the array. Current technology in semiconductor fabrication can only produce arrays in which the diodes are spaced in the neighborhood of 10-100 microns and, as mentioned above, recording requires data spacing down to 0.5 microns. The laser diodes can not be de-magnified optically because of the large numerical aperture of the laser emission. Consequently, to achieve the required density of raster lines on the recording surface, a non-optical method is required to reduce the effective raster line spacing. Such methods normally include one of two techniques: angled diode arrays and interleaving.
An angled diode array is depicted in FIG. 1-A. The diode array 10 is maintained at an angle .theta. with respect to the recording surface 6. Diode spacing d is typically between 10 and 100 .mu.m on the array, but because the array is angled, the spots (r=1-5) which are printed in the tracks on the recording surface 6 are more closely spaced with separations of Y=dcos .theta.. Printing the data onto the recording surface 6 in a linear fashion requires that the diodes of the angled array 10 be activated at delayed intervals. This delay architecture is depicted in FIG. 1-B. The desired location of the printing dots (r=1-5) is in a line on the printing surface 6. Because the printing surface 6 is scanning (i.e. moving relative to the laser diode array 10) in direction 7, the various lasers must be delayed so that they are not activated until the desired location (r=1-5) on the printing surface 6 is reached. Diode n=5 is not delayed, and data is fed straight into it. However, data flowing to diode n=4 must be delayed slightly until spot r=4 is directly under diode n=4. The required delay D is easily determined from the diode spacing d, the array angle .theta. and the scan velocity (not shown). The delay required for the other diodes n=1, 2 and 3 is simply a multiple of that required for n=4. Using this technique of coupling the angled diode array with digital delays, the effective raster line spacing Y can be reduced on the recording surface overcoming the diode spacing limitation of semiconductor fabrication technology.
A second method of overcoming the diode spacing limitation involves interleaving. Interleaving comprises discrete, precise movements of a diode array, such that at each discrete diode array location the recording occurs only on a limited number of raster lines. As the diode array is moved to subsequent discrete locations, recording occurs between the previously recorded raster lines. The interleaving process is extended until all of the tracks have been recorded upon. An interleaving process is thoroughly explained in U.S. Pat. No. 4,900,130 (hereinafter '130), which is hereby incorporated by reference. The following is a brief explanation of the interleaving process as described in the '130 patent.
In the discussion herein of the prior art and of the present invention, certain elements of the invention are referred to by letters. The letters and the elements they refer to are as follows:
d-center-to-center spacing of the array elements (i.e. diodes) or of their images on the recording medium; PA0 N-number of array elements; PA0 n-index number of an element in an array; PA0 p-number of a position of the array; PA0 S-step size of the array; PA0 r-index number of a parallel track; PA0 Y-spacing between parallel tracks on the recording medium ("effective track spacing"); PA0 k-an integer called the "interleaving factor", that is, the number which determines the number of tracks interleaved into a given set of parallel tracks that is recorded at a particular array position; and PA0 D-delay for an element n expressed in the number of positions of the array. PA0 (a) selecting a set of active elements from within the array of individually addressable elements. The selected set of active elements is functional and all the non-activated elements are non-functional; PA0 (b) moving the array so as to produce a set of raster lines in a set of parallel tracks on the recording surface. The set of raster lines is formed only by the elements in the set of active elements as the non-activated elements are non-functional; PA0 (c) moving the array in predetermined discrete steps transverse to the parallel tracks; PA0 (d) repeating the steps (b) and (c) so as to create an interleaving pattern. The interleaving pattern is structured to: PA0 (a) testing the primary elements, thereby determining a set of functional primary elements and a set of failed primary elements; PA0 (b) selecting and activating secondary elements, corresponding to the set of failed primary elements, such that the activated secondary elements may record data in particular tracks corresponding to those of the failed primary elements; PA0 (c) moving the array relative to the recording surface so as to produce a first set of raster lines within a first set of parallel tracks on the recording surface. The first set of raster lines corresponding to the set of functional primary elements; PA0 (d) moving the array a constant predetermined distance relative to the recording surface in a direction perpendicular to the parallel tracks. The predetermined distance is calculated so as to: PA0 (e) moving the array relative to the recording surface, thereby: PA0 (f) repeating steps (d) and (e) in such a manner that:
An array of N elements can expose tracks on a recording surface of effective track spacing Y, which is a fraction of the spacing d of the array elements, by translating the array a constant, discrete step size S. Typically, an array step size: EQU S=Nd/k (1)
is selected, provided that the lowest common multiple of N and k is Nk. If N and k have common factors, then the interleaving will produce multiple exposures on some tracks and skipping of others. With the step size S specified by equation (1), the effective track spacing Y is given by: EQU Y=d/k (2)
Although not a necessary condition, it is advantageous to select N to be prime so as to ensure the greatest possible range of track spacings.
Several implementations of the '130 interleaving process are described in FIG. 2. FIG. 2-A involves an array of N=5 equally spaced diodes with an interleaving factor of k=2 and the spacing of the diodes in the array is shown as d. As indicated in FIG. 2-A, the effective track spacing (given by equation (1)) is Y=d/2 providing a resolution improvement proportional to the interleaving factor k over the actual diode spacing d. The step size of the diode array is given by equation (2) as S=5d/2. The array elements (diodes) are designated n=1,2, . . . 5 (designation not shown in FIG. 2). Elements n=1 and n=2 are not activated at the first array position p=1 which forms the first set of tracks and, consequently, are depicted as clear dots. Similarly, at the last position of the image, certain of the elements will not be activated (i.e. the elements not activated will be in reverse order of non-activated elements at the start of the image). Thus, elements n=4 and n=5 would not be activated on the last pass of the laser diode array. FIG. 2-B depicts the same diode array N=5 and diode spacing d, with an interleaving factor of k=4. As can be seen from the diagram, the spacing of the raster lines is further reduced to Y=d/4 and the step size required is S=5d/4. In FIG. 2-B, element n=1 is not turned on for array positions p=1,2, or 3. Similarly, element n=2 is not activated for p=1 or 2 and element n=3 is not turned on for p=1.
In general, raster line r spaced Y=d/k from an adjacent track is written by element n in an N element array at array position p according to: EQU r=Np-k(N-n) (3)
Note that element n=N always writes raster line r=Np on pass p, regardless of interleaving factor k. This equation can easily be verified by examining FIGS. 2-A and 2-B.
Equation (3) can be used to generate a condition for which diodes will be inactive. An element n is inactivated at position p of an N element array if: EQU n-N-Np/k (4)
Applied to FIG. 2-A, equation (4) indicates that for p=1, diodes n=5/2 will be inactivated. As shown in the diagram, n=1 and 2 are inactive for p=1. For p=2, equation (4) gives n=O and as shown in the diagram, none of the diodes are inactivated. Similarly for FIG. 2-B, for p=1,2 and 3, equation (4) yields n=3.75, 2.5, 1.25 respectively. Accordingly, as can be seen from the diagram, diodes n=1,2 and 3 are inactive for p=1, diodes n=1 and 2 are inactive for p=2 and diode n=1 is inactive for p=3.
To further reduce the effective track spacing on the recording surface a method can be adopted that combines the angled technique described by FIGS. 1-A and 1-B with the interleaving technique of FIGS. 2-A and 2-B. Such a technique requires incorporating the delay networks of the angled technique with the precise algorithms of the interleaving technique. This combination is of little practical difficulty because each technique may be independently implemented without affecting the other.
Another significant problem associated with diode arrays and their use in recording is the failure rate of the diodes. Moreover, if any of the diodes in an array fail, then the entire array is ruined and can no longer be used as a recording means. A need exists to overcome isolated failures of single diodes within the array, so that the array may still function.
Accordingly, it is an object of this invention to provide a fault tolerant diode array recording system which is capable of overcoming isolated diode failures within a diode array, so as to effectively record data onto a recording surface.
Another object of this invention is to provide a laser diode recording system that does not sacrifice resolution (i.e. effective track spacing) in order to achieve its goal of overcoming isolated diode failures within the diode array.